Energy absorber

ABSTRACT

The invention described is a system for seismic isolation or vibration of structures which is simple, relatively inexpensive and readily used for smaller structures. It involves use of bearings comprising a ball ( 10 ) and ball seats ( 12, 18 ) interposed between the structure and its foundation. Either the ball itself ( 24 ) or the seat surface ( 15 ) on which it rolls is compressible, the resistance to that compression providing a frictional braking force resisting rolling of the ball. This serves to dampen relative movement between the structure and its foundation. There are described several alternative constructions of the friction ball which is the preferred central element in the bearing device. There may be a plurality of balls ( 44, 47 ) in each combination of balls and seats.

TECHNICAL FIELD

This invention relates to a frictional braking device. More particularlyit relates to a friction ball primarily intended to be used in seismicand vibration isolation.

BACKGROUND ART

Seismic isolation involves isolating structures and damping the isolatedstructures from incoming forces due to earthquakes. New Zealand Patent178949 describes a very successful bearing which may be used to isolatestructures in this way. The favoured embodiment of this invention isreferred to as a lead rubber bearing. A more comprehensive discussion ofseismic isolation and the lead rubber bearing can be found in “AnIntroduction to Seismic Isolation”, Skinner et, John Wylie and Sons(1993).

Another means of seismically isolating structures is a bearing soldunder the trade mark “Frictional Pendulum” by Earthquake ProtectionSystems Inc, of Alameda, Calif., USA. Such a bearing consists of apendulum member with a frictional surface at the distal end of thependulum member which slides over a polished surface. This is describedin U.S. Pat. No. 4,644,714.

Another device intended to provide seismic isolation is described in theU.S. Pat. No. 4,726,161. This apparatus includes a rollable bearing anda bowl. A spring urges the rollable bearing into contact with the bowl.The combination dampens seismic forces on a structure supported on saidrollable bearing.

In U.S. Pat. No. 5,599,106 there is described a seismic isolatingbearing. This consists in upper and lower load plates with downward andupward facing rigid surfaces, respectively, at least one of which isconical. A rigid ball is sandwiched between the plates. The ball rollsup a conical surface when one load plate is displaced laterally from theother by an external force, and returns to the centre of the core bygravitational forces when the external force ceases.

The invention described in WO97/48866 is a seismic isolator consistingof a ball interposed between a foundation and a building. The ball isretained in a cage like structure secured to the underside of thebuilding. It rolls on a hemispherical surface secured to the foundation.

In all of the prior art devices described above the bearing element, orball, and the surfaces on which it rolls are substantiallyincompressible. This means that when there is relative movement betweenthe structure being supported and the surface upon which the bearingrests there is no braking force being applied apart from the weight ofthe structure itself. If the incoming forces causing relative movementare long lasting then it can take a considerable amount of time for thestructure to come to rest.

It is an object of this invention to go some way towards overcoming thisdisadvantage or at least to offer the public a useful choice.

DISCLOSURE OF INVENTION

Accordingly the invention may be said broadly to consist in an externalforce damping bearing assembly comprising:

an upper surface which in use supports a structure,

a lower surface co-operable with said upper surface, and

a friction ball which in use is positioned between said upper surfaceand said lower surface to support said structure,

said friction ball comprising a core of particulate material and anouter layer enclosing said core, said ball, when supporting saidstructure, being deformed, said deformation generating frictional forcesbetween particles of said particulate material which resists rollingmotion within said ball to dampen an external force applied to saidlower surface or to said upper surface.

In one alternative said external force is a seismic force acting on saidlower surface.

In another alternative said external force is a wind force acting onsaid upper surface through said structure.

Preferably said particular material is a plurality of small frictionballs of construction as defined herein and of diameters sufficientlysmall that a plurality of said friction balls fit within said core.

More particularly said particulate material is sand, quartz, glass,steel, metal balls or spherical or non-spherical balls.

Preferably said core is partly filled with material.

Alternatively said core is completely filled with material.

In a third alternative said core is filled to a hydrostatic pressure of0-5 MPa.

Alternatively said core is filled to a hydrostatic pressure of up to 20MPa.

Alternatively said core is filled to a hydrostatic pressure of up to thestructural limit of said outer layer.

Preferably said outer layer comprises rubber.

More preferably said rubber outer layer is reinforced rubber.

Preferably said rubber is reinforced with fibre, metal wire, carbonfibre, cord, rubber thread or the like.

In another embodiment said friction ball has a laminated constructioncomprising two or more laminae surrounding a central core.

In one embodiment the outer surface of said friction ball is anelastomer with a high coefficient friction.

In another embodiment said outer layer is rubber with a tread.

In another embodiment said outer layer has a treadless but roughenedsurface.

Preferably the shape of said friction ball is any geometrical shapecapable of rolling motion.

Preferably the undeformed shape of said friction ball is spherical.

Alternatively said undeformed shape is elliptical.

Alternatively said undeformed shape is ellipsoidal.

Preferably at least one of said upper and lower surfaces is concave orconical.

More preferably both said surfaces are concave.

Preferably said concave surface may be spherical, parabolic or definedby a polynomial, trigometrical or hyperbolic function or any othersmooth curve or combination of curves.

Alternatively said surface is conical.

Preferably there is a rest in said concave or conical surface adapted toreceive a said friction ball.

Preferably said rest is concave with a radius of curvature less thanthat of said concave surface.

Preferably a said lower surface mountable on a foundation member.

Preferably said foundation member is a pile.

More preferably said foundation member is an internal or externalbasement wall.

Preferably a said upper surface is mountable on the bottom of astructure to be supported thereby.

Alternatively said upper surface is an integral part of said structure.

In a further alternative there are a plurality of said friction ballsresting on each said lower surface.

In one embodiment there are three said friction balls.

In another embodiment there are seven said friction balls.

Preferably there is a rest within said lower concave surface to receivesaid plurality of friction balls, the rest being shaped so that theupper surfaces of said balls contact said upper surface evenly over thecontact area of each of said balls.

In another embodiment the invention may be said broadly to consist in amethod of seismically isolating a structure which comprises interposinga plurality of bearing assemblies (as herein above defined) between astructure and a predetermined number of foundation points upon whichsaid lower surfaces are positioned.

In one alternative said bearing assemblies are installed at the time ofconstruction of said structure.

In another embodiment said bearing assemblies are retrofitted between anexisting structure and its foundation.

Preferably said bearing assemblies are provided above all foundationmembers.

Preferably said foundation members are piles.

Alternatively said foundation members are perimeter or internal walls.

Preferably said structure is a house.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more of said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

The invention consists in the foregoing and also envisages constructionsof which the following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a structure, a surface and a ballillustrating their relative motions on a theoretical basis.

FIG. 2 is a schematic sectional view of a ball interposed between a pairof seats.

FIG. 3 is a sectional view of one embodiment of a friction ballaccording to the invention at rest and deformed by the weight of astructure resting thereupon.

FIG. 4 is a schematic view of concave upper and lower seats according toa preferred embodiment of the invention.

FIG. 5 is a schematic sectional view of upper and lower seats accordingto an embodiment of the invention with a pair of rests.

FIGS. 6 and 7 are cross-sectional views of laminated embodiments offriction balls according to the invention.

FIG. 8 is a schematic diagram of a structure, a seat and a ballaccording to the invention in which the frictional deformation isprovided primarily in the seat component.

FIG. 9 is a plan view of a lower seat on which three friction balls areprovided.

FIG. 10 is a plan view of a lower seat on which seven friction balls areprovided.

MODES OF CARRYING OUT THE INVENTION

One of the practical impediments to inserting bearings under smallerstructures to isolate them seismically is the need to allow forsufficient travel of the structure in relation to the foundation so thatdamping can take place. In FIG. 1 there is illustrated a lower seat 12with a curved upper surface 14 upon which a ball 10 rests. For thepurposes of this illustration ball 10 is not deformed, whereas in theembodiment according to the invention ball 10 is deformed. A structure16 or a portion of a structure 16 rests on top of ball 10. As structure16 is moved from left to right to the position shown in ghost in FIG. 1,ball 10 moves to the position 10 a up the curve of surface 14 of radiusR. Because each of the top and bottom of ball 10 is in contact with asurface the net effect on member 16 is that it will have movedhorizontally a distance 2Δx as illustrated at the same time that ball 10a has moved horizontally a distance Δx and vertically a distance Δy. Themechanical advantage achieved by a rolling ball is used in the conceptof a friction ball according to the invention to allow for relativemovement between a structure and its foundation during incoming forcesdue to earthquakes.

In FIGS. 2 and 3 there is illustrated a friction ball 10 according tothe invention. In FIG. 2 it is shown in an undeformed condition. In FIG.3 it is shown in a deformed condition which it would assume when astructure was resting upon it. In the embodiment of FIGS. 2 and 3 thefriction ball consists of a central core 22 and an outer layer 24.

A vast number of alternative materials can be used in the constructionof the friction ball. The first property of the friction ball whichallows it to be used in seismic isolation is that it must be deformableunder the weight of a structure which it is intended to isolateseismically. The second property is that in the condition of deformationfrictional forces within the friction ball are enhanced so as to resistrolling motion.

In the construction illustrated with reference to FIGS. 2 and 3, theouter layer 24 is preferably made of an elastomer, most preferablyrubber. Its purpose and function is very similar to that of a tire treadon a vehicle tire. Thus, materials which are suitable in theconstruction and reinforcement of tire treads are equally suitable tothe outer layer 24. The outer layer 24 may be provided with a tread orit may be treadless but with a roughened surface to enhance frictionbetween the surface and the seats upon which it will rest. The outerlayer may be reinforced with materials such as various fibres, metalwire, carbon fibre and rubber thread.

The core 22 can consist of a number of materials. Preferably it consistsof particulate material such as sand, quartz particles, glass beads,steel balls, concrete balls or metal balls. The material can be ofregular geometric shape or of non-geometric shape. The main criterion isthat when the friction ball 10 is deformed the particulate matter iscompressed in such a way that the interparticle frictional forces areincreased.

In one preferred alternative embodiment the particulate material in core22 can be a plurality of friction balls. The friction balls may be ofthe same construction as is described hereinabove. That is they can besolid or a core surrounded by an outer layer or laminated. However, foruse as particulate material they will of course have a much smallerdiameter than the friction ball made up of a plurality of friction ballswithin the core. The friction balls may be of the uniform or non-uniformdiameter and shape so long as they meet the other criteria describedherein.

The core 22 can be of a compressible solid material such as lead,concrete, wood, rubber or metal which in conjunction with thedeformation of the outer layer would increase frictional forces thusresisting rolling of the ball.

The core may be partly filled or totally filled to hydrostatic pressuresup to, for example, 20 MPa. It will be appreciated that the pressure onthe core will be increased when it is deformed by the weight of thestructure resting upon it and the outer layer 24 should be constructedaccordingly.

In one embodiment the friction ball may be of a homogeneous constructionof for example, solid rubber or other elastomer.

The shape of the ball is not critical provided it is capable of rollingbetween a structure and a seat. The simplest form of ball is spherical.However, it could be elliptical (an ellipse in one plane and circular inthe other), ellipsoidal (ellipse in two planes with the same ordifferent major and minor axis) or other similar shape. The ball mayhave any smooth or non-smooth surface to give the required forcedisplacement function. The ball may have a flat or a flattened surfaceand a rough or a smooth surface.

Referring to FIGS. 6 and 7, friction balls according to the inventionmay consist of a core 22 and an outer layer 24 and interposedtherebetween a series of layers 23, 25, 27 and 29. Each of theinterposed layers may be of solid or particulate material and may or maynot be deformable provided that the overall structure is deformable andin the state of deformation increases resistance to rolling over that ofundeformed friction ball.

Friction balls may be made of relatively inexpensive materials. This isof particular advantage in under developed countries which are subjectto earthquakes. A friction ball could be made by filling the inside of asoccer ball with sand. Alternative relatively inexpensive materials suchas animal skins can similarly be filled with granular materials such assand or gravel in such circumstances.

In FIGS. 4 and 5 there are illustrated seats in which the friction ballwill be positioned. In FIG. 4 a representative ball 10 (which would bedeformed when a structure is placed on upper seat 18) is positionedbetween an upper seat 18 and a lower seat 12. The upper face of ball 10rests against the curved lower surface 20 of upper seat 18. The lowerface of ball 10 rests on the curved upper surface 14 of lower seat 12.

The surfaces 20 and 14 of the seats 18 and 12 respectively can be of anycurved shape such as spherical, parabolic or the like. It can also beconical in shape, the conical shape being particularly helpful in selfcentering friction balls when they come to rest.

In the embodiment in FIG. 5 a seat 16 having a substantially flat lowersurface 19 has a rest or curved indented portion 26. An opposing seat 12with a concave upper face 14 also has a curved indented rest 28. Whenthe friction ball 10 is in the rests 26 and 28 the structure is in thedesired centred position. In the preferred embodiment the friction ball10 will be sufficiently deformable that it will deform to the shape ofthe rests 26 and 28 when in the centred position as described.

The rolling resistance frictional forces used to create seismicisolation according to the invention can be applied by deformationoccurring in either the frictional ball or in the seat. Referring toFIG. 8 in such an embodiment a seat 12 having a substantially sphericalsurface 14 is provided with a deformable layer 15. Layer 15 is made ofdeformable material as described in relation to a friction ball in theother embodiments of the invention. In the embodiment of FIG. 8 a ball11 is substantially non-deformable. It deforms the layer 15 to a depthshown by the dotted line 17. During relative movement between the seatand the structure 16 resting on ball 11 as it moves to position 11 a.The frictional forces within the layer 15 resist the rolling movement sothat the structure 16 moves only the distance Δx shown in FIG. 8 beforethe frictional resistance stops rotation of the ball in position 11 a.

When the friction ball according to the invention is to be used inseismic isolation the points where the structure to be seismicallyisolated rests on its foundation are selected. Preferably upper andlower seats and a friction ball are installed at each such site. It ispossible to install the friction ball on one seat only but it ispreferred that there be a seat at either side. It is also preferred thatthe seat on either side has a rest 26 and 28 as shown in FIG. 5. Theseats may be made of any suitable solid material such as mouldedconcrete or wood or the like.

If a structure is displaced in such a manner that one or more frictionballs do not return to the rests 26 and 28 the building can be movedmechanically to return it to its centred position.

A further variation of the invention is the provision of a plurality ofballs between the upper and lower seats of a ball and seat combination.These are illustrated in FIGS. 9 and 10.

The seat 40 in each of FIGS. 9 and 10 is similar to the lower seatillustrated in each of FIGS. 4, 5 and 8. There is provided an indentedrest 42 (similar to rest 28 in FIG. 5) having a radius of curvaturewhich is less than that of the concave upper surface of seat 40. In FIG.9 there are three friction balls 44 in the indented rest 42. In FIG. 10there are seven such balls 46 and 47. The outer six balls (each numbered47) surround a central ball 46. Because of the packing of the spheres onthe surface of rest 42 central ball 46, although of the same diameter asballs 47 will have its upper surface slightly elevated. To compensate afurther rest similar to rest 28 in FIG. 5 may be provided in the centreof rest 42.

It is preferable that the upper surfaces of each of balls 44, 46 and 47each bear the same amount of weight of the structure or upper seat theyare supporting. Because either the balls themselves or the surfaces onwhich they rest are deformed when supporting the weight of the buildingthis feature is not essential but it is preferable. Because either theballs or the surfaces on which they rest are deformed when supporting aweight if there is unequal distribution of the load between the ballsthis will be compensated for by the extra deformation of either the ballitself or the surface on which it rests.

For this reason it is not essential that there be an indented rest butthis feature is advantageous.

The number of balls in the plurality is a matter of choice but couldinclude from 2 to an upper limit determined by the size of the balls andthe seats. Some assistance in choosing the number is to use themathematical theories of close packing of spheres. Three and seven ballsare preferred numbers, but the invention is not limited to these.

There are advantages in having a plurality of balls. Because the weightto be supported is distributed over a greater number of supportingmembers the devices can be used with heavier structures resting on themthan if single balls were used. It is also possible to achieve astandardisation of components. Friction balls of a single size can beused in any application with any weight of structure. Where there was aheavier structure being supported there would be a greater number ofballs doing the supporting.

In a domestic dwelling the sites chosen for installation may be piles orinternal or perimeter foundation walls.

Because the concept of seismic isolation involves relative movementbetween the structure and the foundation it is necessary that a gap isleft between the structure and the foundation and that there issufficient room left for the travel of the structure relative to thefoundation when seismic forces are acting upon the foundation beneaththe structure.

When an array of friction balls are installed between a structure andits foundation and seismic forces hit the foundation the relativemovement between the structure and the foundation will be occurringwhile the ball itself is deformed. The frictional forces within the ballwill resist rolling and decelerate the relative movements before theball has moved laterally beyond the edges of the seat. The relativemotions will continue as long as the earthquake continues and therolling resistance of the deformed frictional ball will continue todampen the forces in the manner described.

The natural period of isolation in a particular direction is

2π{square root over (R/g)}

where ‘R’=radius of curvature of the curved surface in that directionand at that position, and

‘g’ is the acceleration due to gravity.

It will be appreciated by those skilled in the art that in designingfriction balls the resistance to rolling of the friction ball in theconditions to be encountered when it is installed must be calculatedsuch that the ball does not roll horizontally out of its seat and thestructure collapse upon its foundation. The manner in which this iscalculated is, for friction balls filled with particulate material andnot prestressed, the coefficient of friction μ, for friction, coulomb,or for plastic devices,

 μ=4 tan ⊖

where ⊖ is the angle between an perpendicular line from the frictionball centre to the surface of deformation and the end of the deformationwhere the ball resumes its natural curvature as shown in FIG. 3.

Where the friction ball is made of solid material subject to elasticdeformation, the elastic material theories described in H R Hertz,“Miscellaneous Papers” (McMillan, London 1896), Chapters 5 and 6 can aidin determining the coefficient of friction for a friction ball.

For plastically deformable solid friction balls this may be determinedby the method described in Robinson and Truman, “Stress-Strain Curve forAluminium for a Continuous Indentation Test”, Journal of MaterialsScience 12 (1977), 1961-1965.

The invention has been described in relation to the use of frictionballs for seismic isolation of structures from their foundations.

The friction balls can also be used in isolating different portions ofthe same structures or different structures from each other. Forexample, friction balls may be used wherever friction may be of service.One such example would be in rotating machinery where friction balls maybe used as brakes or clutches.

Friction balls or deformable seat features according to the inventionmay be used in other applications. For example, they may be used in theseismic isolation of small industrial buildings. They may be used fordamping vibrations owing to other sources. For example, they may be usedto isolate machinery from structures in which the machinery is situated.

Other permutations or combinations within the scope of the invention asdefined will also be apparent to those skilled in the art.

What is claimed is:
 1. An external force damping bearing assembly for astructure, said assembly comprising: an upper surface which in usesupports the structure, a lower surface co-operable with said uppersurface, and a friction ball which in use is positioned between saidupper surface and said lower surface to support said structure, saidfriction ball comprising a core of particulate material and an outerlayer enclosing said core, said ball, when supporting said structure,being deformed, said deformation generating frictional forces betweenparticles of said particulate material which resists rolling motionwithin said ball to dampen an external force applied to said lowersurface or to said upper surface.
 2. The bearing assembly as claimed inclaim 1, wherein said external force is a seismic force acting on saidlower surface.
 3. The bearing assembly as claimed in claim 1, whereinsaid external force is a wind force acting on said upper surface throughsaid structure.
 4. The bearing assembly as claimed in claim 1, whereinsaid particulate material is a plurality of small friction balls of aconstruction and of diameters sufficiently small that a plurality ofsaid friction balls fit within said core.
 5. The bearing assembly asclaimed in claim 1, wherein said particulate material consists of atleast one of the following: sand, quartz, glass, steel, metal balls,spherical balls and non-spherical balls.
 6. The bearing assembly asclaimed in claim 1, wherein said core is filled to a hydrostaticpressure of up to 20 Mpa.
 7. The bearing assembly as claimed in claim 1,wherein said outer layer comprises rubber.
 8. The bearing assembly asclaimed in claim 7, wherein said rubber outer layer is reinforcedrubber.
 9. The bearing assembly as claimed in claim 1, wherein saidfriction ball has a laminated construction comprising two or morelaminae surrounding a central core.
 10. The bearing assembly as claimedin claim 1, wherein said friction ball is spherical when it is notdeformed by the weight it supports.
 11. The bearing assembly as claimedin claim 1, wherein at least one of said surfaces is concave or conical.12. The bearing assembly as claimed in claim 11, having a rest in saidconcave or conical surface or surfaces adapted to receive said frictionball.
 13. The bearing assembly as claimed in claim 1, wherein said lowersurface is mountable on a foundation member.
 14. The bearing assembly asclaimed in claim 1, wherein there are a plurality of said friction ballsresting on said lower surface.
 15. A method of seismically isolating astructure said method comprising the steps of: interposing a pluralityof bearing assemblies, using an upper surface to support the structure,using a lower surface co-operable with said upper surface, positioning afriction ball between said upper surface and said lower surface tosupport said structure, and providing said friction ball with a core ofparticulate material and an outer layer enclosing said core, said ball,when supporting said structure, being deformed, said deformationgenerating frictional forces between particles of said particulatematerial which resists rolling motion within said ball to dampen anexternal force applied to said lower surface or to said upper surfacebetween the structure and a predetermined number of foundation pointsupon which said lower surfaces are positioned.